Add customers to a campaign

Add customers to a campaign so it starts reaching out to them. Requires campaign_enrollment.manage. Provide customer or customers (up to 1,000), identified by customerId or US phoneNumber. Returns HTTP 202 with a server-generated job ID for both individuals and lists. Invalid customer entries fail individually with invalid_customer_reference and validationErrors; valid entries continue. Invalid request structure still returns 422. Poll statusUrl for per-input results. An optional idempotencyKey protects retries of the same payload; without one, each submission creates a new request. Customers must already exist in your organization. See Campaign API guide for restrictions and retries. Customers already in the campaign, or who already finished it, are not started over. A customer can be in more than one campaign at a time.